Field measurement and modeling of cowpea water use and yield under stressed and well-watered growth conditions

A two-year field study was conducted to characterize the effect of water deficit imposed during one or more growth stages on the production of cowpeas (Vigna unguiculata (L.) Walp). Plant growth, plant-water potential, soil-water status, and weather parameters were measured. Five different models for potential evapotranspiration were compared with measured evapotranspiration from a well-watered crop during the full cover stage. Only those methods which included a vapor-deficit correction for advection effectively described potential evapotranspiration in southern California. A model for evaporation and transpiration under stress and growing cover conditions was calibrated in one year and tested in the subsequent year and found to be in good agreement with measurements taken by a hydrologic balance. Predawn xylem pressure potential was found to decrease as the soil-water potential decreased during a drying cycle. The midday xylem pressure potential was not well correlated with soil-water potential, and wilting was never observed. Cowpea dry-matter production under limited water conditions was linearly related to crop water use and relatively insensitive to the timing of the water deficit. However, a model to predict dry matter yield from primary water balance and atmospheric measurements produced only fair agreement with measurements during the year of testing.